A "B" is GOOD, that's what it shows on the report card. I think it's OK to encourage all A's as long as you're not obsessive with it. I encourage A's and B's. My daughter makes mostly A's and some B's, no C's .. My son started out that way, but around 8th grade, the C's started showing up. I encouraged him to work harder, but never overreacted and I never punished him. Why? Because, as a parent, I have always compared myself as an adult/parent and how I was as a kid, that was once my child's age.
My parents would make me stay in the house for a month if I came home with a "C". When I was younger, if I made a "C" I would get a whipping, plus have to stay in the house for weeks. Long story short, that didn't make me a smarter student, didn't make me a better man. I tried harder, and in most cases, improved the grade, but in the long run their punishments had little effect on my life.
I didn't become a physician, not the president of the US. Oftentimes parents push kids too hard because they want to live vicariously through their children. A "B" is GOOD.